After that I gave the satellite imagery to Simon Kokoyo form COOPI and Reality Tested Youth Program. He walked around the slum and talked to elders, who helped him determine the village boundaries of the slum by drawing them on the printed satellite imagery.

Drawn villages of Mathare

I uploaded the satellite imagery into JOSM and digitized the boundaries according to the drawn image.

Digitizing in Java OpenStreetMap Editor

And HERE are the villages of Mathare and Mathare slum area in OpenStreetMap – the borders have already changed couple of times